Structural composites are used in a wide variety of applications that require plastic parts having a minimum threshold of mechanical properties such as strength and impact resistance. Composites are typically made by introducing glass, or other reinforcing fibers, to a thermoplastic or thermosetting polymer material. The glass fiber and polymer material are mixed together and formed into a composite part in a wide variety of methods, including compression and injection molding. Examples of structural composites include sheet molding compound (SMC) and structural reaction injection molding (SRIM).
Structural composites are formed using glass reinforcements that provide dimensional stability and excellent mechanical properties to the resulting composites. For example, glass fibers provide dimensional stability as they do not shrink or stretch in response to changes in atmospheric conditions. In addition, glass fibers have high tensile strength, heat resistance, moisture resistance, and high thermal conductivity. It is known in the art that glass fiber-reinforced polymer composites possess higher mechanical properties compared to unreinforced polymer composites, provided that the reinforcement fiber surface is suitably modified by a sizing composition. Thus, better dimensional stability, tensile strength and modulus, flexural strength and modulus, impact resistance, and creep resistance may be achieved with glass fiber-reinforced composites.
Typically, glass fibers are formed by attenuating streams of a molten glass material from a bushing. An aqueous sizing composition, or chemical treatment, is typically applied to the fibers after they are drawn from the bushing. After the fibers are treated with the aqueous sizing composition, they may be wound onto a collet, packaged, dried, and wound together into a continuous roving. The roving may subsequently be chopped for use in a molding process for making a structural composite.
Conventional sizing compositions typically contain one or more film forming polymeric or resinous components, glass-resin coupling agents, and one or more lubricants, emulsifiers, and/or antistatic agents dissolved or dispersed in a liquid medium. The film forming component of the size composition is desirably selected to be compatible with the matrix resin or resins in which the glass fibers are to be embedded. The sizing composition protects the fibers from interfilament abrasion and promotes compatibility and adhesion between the glass fibers and the matrix in which the glass fibers are to be used.
Numerous problems exist with existing technologies used to make composite parts and with the composites themselves. For example, the thermoplastic and thermosetting polymer materials commonly used in these systems are solvent-based systems. As a result, volatile organic compounds (VOC's) may be released into the atmosphere as the part is cured. One potential polymer system that may be used to reduce the amount of volatile organic compounds that are emitted into the environment and provide a friendlier workplace is a powder coating system. Powder coating is a coating that is applied as a dry powder to produce a colored surface finish on a part. As a result, such a powder coating system releases little or no volatile organic compounds.
Additionally, in conventional compression and injection molding processes, the resin and the glass fibers are generally combined in a separate step prior to molding, which adds to the manufacturing costs. For instance, in a typical SRIM process, the reinforcement fibers and polymer resin are introduced individually to molding equipment, after which, the part is molded. In these types of molding techniques, composite parts may be formed that have inadequately dispersed reinforcement materials if the machine is not calibrated properly. Further, the amount of fiber content that may be introduced in injection and compression molding processes is limited due to the process itself, as is known in the art. Because conventional molding processes contain a lower fiber content and because matrix resins are more generally expensive than the reinforcement fibers, the cost of the composite part is increased.
In the production of structural composites, the sizing composition should be highly soluble so that the individual fibers are sufficiently dispersed or wetted by the matrix resin. The high solubility promotes better fiber strand defilamentation, or strand break up, which reduces fiber prominence and thus improves the uniformity or smooth appearance of the surface of the resulting composite. Additionally, the high solubility promotes an increased interface between the individual fibers and the matrix resin and results in better mechanical properties, which are needed for structural applications.
It is desirable that the strands remain in a bundled formation until the proper time for defilamentation. Accordingly, a need exists in the art for an improved size composition that exhibits a high solubility, is easy to manufacture, and is easily applied to glass fibers. It is also desirable to provide a method for combining the powder coating industry and glass fiber industry for making structural composite parts, to increase the fiber content in structural composite parts to improve or maintain mechanical properties such as strength or impact resistance, and to simplify the manufacturing of structural composite parts by introducing the polymer resin and fiber reinforcement material in one step to be molded to a desired shape.